Terahertz spectroscopy system and method

ABSTRACT

A terahertz spectrometer includes: a terahertz-wave emitter and a terahertz receiver elements. The terahertz wave generated by means of generating beat frequency corresponding to the difference between two rapidly tunable continuous wave lasers. Having a difference in time between the interrogating signal and the reference signal at the receiver end side, which corresponds to intermediate frequency (IF), not centered around the baseband, i.e. zero Hertz. The offset step size of the intermediate frequency from zero Hertz is linearly correlated to the position of the interrogated object position.

FIELD OF THE INVENTION

This invention relates to spectroscopic system and method andparticularly to terahertz spectroscopy techniques.

BACKGROUND OF THE INVENTION

Spectroscopy is one of the main compelling applications of terahertz(THz) radiation. A typical THz Spectroscopy system includes a tunableTHz transmitter capable of generating THz radiation for irradiating asample and a THz detector capable of receiving THz radiation responsefrom the irradiated sample and providing indication (electric signal) ofthe strength and propagation delay of the detected radiation from thesample. For example, the technique of obtaining information related toterahertz waves that are transmitted through or reflected by a sample isdisclosed in U.S. Pat. No. 7,551,269.

A tunable THz transmission device typically includes two distributedfeedback (DFB) lasers and a THz emitter associated with an antenna. Oneor both of the lasers are associated with a controllablethermo-electric-cooling (TEC) system which controls their operatingtemperatures and thus their output wavelengths. The lasers are used toilluminate the THz emitter (being typically a photo-conducting element)with a light signal containing an oscillating component at the beatfrequency (difference frequency) of the lasers. In the emitter, a THzfrequency current is excited while applying D.C. bias to thephoto-conducting element which changes conductance at the beatfrequency, causing the antenna coupled to it to radiate in the THz band.The frequency of the current in the antenna (and that of the emittedradiation) is the difference between the frequencies of the lasers (beatfrequency), and thus tuning the frequency of the emitted THz radiationis achieved by changing the output frequency(ies) of one or both of thelasers. Such photo-mixing based THz emitter is described for example inWO 2007/132459, assigned to the assignee of the present application.

In a THz detection device (receiver), a responding THz radiation signal,(e.g. reflected, transmitted or scattered wave) from the irradiatedsample is incident upon the antenna of the receiving device, which isconstructed similarly to the emitting device. This THz signal induces avoltage across the receiving photo-conductor which, in this case, hassubstantially zero D.C. bias component. The conductivity of thereceiving photo-conductor is also modulated at the optical beatfrequency by the incident laser light in the same way as the transmitterdevice is modulated. If the beat frequency is constant, the THzmodulation of the conductance interacting with the THz bias created bythe signal from the antenna generates a low frequency (e.g. D.C.) signalcomponent proportional to the amplitude of the incident THz wave anddependent on the relative phases of the received wave and the opticalbeat-frequency modulation. Such arrangement acts as a homodyne mixer inwhich the modulating optical beat frequency used in the emitter is alsoused as a reference signal (i.e. reference oscillator modulation) in thereceiver. The intermediate frequency is centered around the D.C. (zerofrequency), and the arrangement provides coherent detection. The desiredsignal centered at D.C. can be extracted by using a low pass filter.

The above is schematically shown in FIG. 1. Two light beams ofwavelengths/frequencies (λ₁, ω₁) and (λ₂, ω₂) respectively are combinedby a fiber splitter/combiner to propagate along a combined optical path,and then are split into two light components, each of a beat frequency(ω₂−ω₁) propagating along spatially separated optical paths towardsrespectively the transmitter- and receiver-antenna units. The lightcomponent at the receiver-antenna unit serves as a reference beam orlocal oscillator modulation. Each of the transmitter- andreceiver-antenna units includes photo-conductors with antennas.Radiation emitted by the transmitter-antenna unit is directed (by areflector) to the sample, and a radiation response of the sample(reflection from the sample) is directed (by another reflector) to thereceiver-antenna unit. The latter includes a low-pass filter whichoperates to extract the desired signal. A photomixing based transceiversystem of the kind described above is disclosed for example in U.S. Pat.No. 6,348,683.

A major disadvantage of such arrangement is associated with the factthat amplifiers exhibit high noise density at low frequencies called“flicker noise”. Accordingly, in order to achieve reasonable signal tonoise ratio, the signal (THs radiation incident onto the sample) has tobe of as high as possible amplitude, and thus enabling the terahertzsignal from the sample to be sufficiently strong when arriving at thereceiver. Since the “flicker noise” and a detected signal (resultingfrom the interaction between the sample's response and referencesignals) are in this case occupy frequency band with high density noise,a band pass filter (low pass filter) cannot be effectively utilized tofilter out the noise. The severity of the flicker noise phenomenon isillustrated in FIG. 2 which shows noise density for a typicalintegrated-circuit amplifier (e.g. utilizing MAX4475 amplifiercommercially available from Maxim. Integrated Products Inc). It may beseen that the noise density is rising rapidly as frequencies approachD.C. At 10 Hz the density is more than five times the density at 10 kHz,and at 1 Hz the density will be very much larger, probably severalhundred time the density at 10 kHz.

Another technique of the kind specified is disclosed in U.S. Pat. No.7,687,773. This technique relates to sub-millimeter wave frequencyheterodyne imaging systems, more specifically, to a sub-millimeter wavefrequency heterodyne detector system for imaging the magnitude and phaseof transmitted power through or reflected power off of mechanicallyscanned samples at sub-millimeter wave frequencies.

GENERAL DESCRIPTION OF THE INVENTION

There is a need in the art in optimizing high-frequency spectroscopy(THz range spectroscopy). The present invention meets this need byproviding novel methods and devices for use in a high-frequencyspectroscopic system enabling to improve signal to noise ratio of thesystem operation.

The main idea of the invention consists of providing a desired (e.g.desirably high) frequency difference between responding radiation comingfrom a sample under inspection and reference radiation when bothsimultaneously arrive to an antenna receiving unit. To this end, theinvention takes an advantage of frequency sweeping that is to be used inspectroscopy. This is associated with the following:

In spectroscopy applications it is common to sweep monotonically thefrequency of the emitted radiation across a certain desired frequencyrange. As indicated above, in cases where two or more laser beams areused for generating a high frequency (THz range) radiation (e.g. byphotomixing), frequency sweeping is carried out by sweeping thefrequency output of at least one laser source, or both of them in apredefined rate(s).

Spectroscopic measurements in the THz regime are performed byirradiating an inspected object with THz radiation and detecting a THzradiation response from the object. In the following description, theTHz radiation irradiating the sample/object is referred to as inspectingradiation, while the part of the inspecting radiation reflected from ortransmitted through the object and detected by the detector/receiver issometimes referred to as responding radiation. As indicated above,typical detection devices for THz spectroscopy use homodyne detection inwhich the responding radiation from the object (sample) is mixed with areference radiation, which has properties corresponding to theinspecting radiation to generate a detection signal (e.g. anelectric/electromagnetic signal). The reference radiation may be forexample a radiation portion, sourced together with the inspectingradiation having similar properties, and transmitted directly to thereceiver/detector (not through the inspected object/sample). Thedetection signal generated by the mixing of the reference radiation andthe responding radiation can be more conveniently processed as it has alower frequency than that of the responding radiation. This enablesdetermination of the spectral properties, determining physical and/orchemical properties/conditions of the object, from amplitude and phasemeasurements, as being functions of frequency of the inspectingradiation, based on detection of the responding radiation.

Thus, the homodyne detection is based on that the reference signal andthe inspecting radiation for irradiating the object are originated bythe same source(s). Reference radiation may be generated in thetransmitter by splitting the laser output, bearing the beat frequencysignal, into two portions. One portion is then used for generation ofinspecting THz radiation and the other for creation of the referencesignal with which to coherently detect the responding radiation.

Generally, at the detector, the instantaneous frequencies of thereference radiation and the inspecting radiation arriving as responsefrom the object/sample are different. This is because of the frequencysweeping carried out in the THz generator (emitter), and because of atime delay between the arrivals to the detector of the respondingradiation (radiation response from the object) and the referenceradiation the origination of which have been concurrently initiated atthe emitter (i.e. have the shared laser source). This time delay may,for example, be a result of different propagation path lengths traversedby the inspecting and reference radiation components to the receiver,and is also associated with a delay in transit of theinspecting/responding radiation due to interaction with the sample.After mixing of the responding radiation with the reference radiation(homodyne detection), and possibly also after suitable band passfiltering, the resultant detection signal has an intermediate frequencycomponent, which is of the order of the frequency difference between theresponding and reference signals and typically has (according to theconventional techniques) low frequencies centered around zero frequency.

It should be noted here that for the purposes of the present applicationthe terms intermediate frequency, intermediate frequency component andhomodyne frequency used herein generally refer to the frequency of asignal resulted from mixing of the detected signal from the sample withthe reference signal/radiation. Also, is should be noted that,differently from typical homodyne detection systems, in which suchintermediate frequencies are generally centered around the zero value,the homodyne detection effect utilized in the invention provides forintermediate frequencies which are not centered around zero Hertz valueand which therefore are less affected by noise effects such as the“flicker noise”.

Let us consider for example the use of a THz generator havingsubstantially constant frequency sweeping rate β for generating theinspecting and reference radiation. Here, β is the time derivative ofthe frequency of the transmitted THz radiation in cycles per second persecond, and is also referred to herein as frequency variation rate. Forsuch THz generator, a frequency difference between the detected responseradiation and the reference radiation is given by β·τ (τ being the timelag between the receipt of the reference radiation and the responseradiation at the detector). In typical THz spectroscopy, β may be of theorder of 1 THz/sec and τ may be of the order of nanoseconds.Accordingly, in this example the frequency difference is 1 kHz pernanosecond, i.e. 1 THz/sec. According to the conventional techniques,this frequency offset effect is compensated by adjusting the referenceand inspection/response path lengths (e.g. through controlled delay ofthe reference radiation component) so that the differential delay isartificially calibrated to zero (τ=0).

As an alternative to adjusting the delay to zero, the spectroscopy maybe accomplished by step-wise excursion of the interrogating frequency(frequency of the radiation produced by the system, i.e. inspecting andreference), rather than continuous scanning. In this implementation, thefrequency is held constant (i.e. β=0) during a measurement interval.However, this constraint imposes quantization on the frequency variablewhich may be undesirable when searching for fine-grain features in thespectral response. In addition, operating in discrete (step wise)frequency methodology, a fairly long transient time for frequencysettling is generally required at each frequency step which contributesto a substantial increase in the time required for a spectroscopicmeasurement of a sample.

The present invention provides for resolving this deficiency, so thatthere is neither a need to match time delays in the arrival of referenceand interrogation/responding signals to the receiver (controlling theoptical paths), as compared to the conventional techniques (e.g.described in U.S. Pat. No. 7,551,269, nor a need to constrain the methodof scanning. The invention permits the use of continuous scanning, whilefacilitates fast scanning and removes the need to match delay paths.

The present invention also provides for exploiting the coupling betweenfrequency change rate and delay to enhance the measurement quality. Thisis achieved by providing higher sweeping rate beyond that needed forbasic spectroscopy.

In order to allow accurate spectroscopic measurements/detection, thefrequency (and possibly also the phase) of the reference radiation,which is mixed with the radiation response, should correspond to the THzfrequency from the THz transmitter. Accordingly, the reference signaland inspecting radiation for irradiating the object are originated bythe same light source; the output of the light source (laser based lightsource) is split into two portions. The first portion is used togenerate THz range radiation which is directed to propagate to thedetector via the interaction with the sample. The second portion isutilized for producing/transmitting reference radiation directly to thedetector.

According to the invention, at the detector, the instantaneousfrequencies of the reference radiation and the inspecting radiationarriving as response from the object are different. This is because ofthe frequency sweeping carried out in the THz generator (emitter), andbecause the path lengths of the inspecting and reference radiationcomponents need not be adjusted to reduce a time delay between theirarrivals to the detector. Accordingly, the inspecting radiation(radiation response from the object) and the reference radiationcomponents that concurrently arrive at the detector are those that hadbeen originated/initiated at the emitter at different times, andtherefore have different frequencies due to the frequency sweepingcarried out in the emitter. This time delay results from the differentoptical path lengths traversed by the reference radiation and theinspecting radiation interacting with the sample. After mixing of theinspecting radiation with the reference radiation (homodyne detection),and possibly also after suitable filtering, the resultant detectionsignal has an intermediate frequency component of the order of thefrequency difference between the inspecting and reference signals whichis in turn proportional to both the time delay and the frequencysweeping rate.

Turning back to the example above and considering a THz generator withfrequency sweeping rate β of the order of 1 THz/sec and a time lag τ ofthe order of nanoseconds between the arrivals of inspecting andreference radiation components at the detector, the frequency of thedetection signal (the beat frequency obtained after mixing theinspecting and reference radiation), is of the order of several KHz,e.g. 5-100 KHz.

It is desired to increase the frequency difference (i.e. theintermediate frequency) between the reference and inspecting radiationcomponents simultaneously arriving to the detector, such that a higherhomodyne frequency (i.e. intermediate frequency) is obtained. This isbecause using larger frequency difference allows higher signal to noisein the detection signal and because the noise density (flicker noise) issmaller for higher frequencies.

Moreover, higher intermediate frequencies are also desired since theyallow improved range discrimination (higher range resolution/depthresolution) of the sample. The range resolution that can be obtained byspectroscopic detection is given by

${{resolution} = \frac{c}{2\beta*\tau}},$where c is the speed of light, τ is a time delay between the referenceand inspecting radiation arriving at the detector and β is the frequencysweeping rate. Improving the range discrimination enables betterfiltration out of noise and sporadic radiation, such as reflections,from the detection signal, thus also enabling to increase the signal tonoise of the spectroscopic inspection. With regard to depth profilingapplication of this invention, it should be noted that in order to get aphase reference (i.e. a location inside the sample corresponding to thedetected response), the free space path is appropriately calibratedprior to actual measurements. The present invention is based on theunderstanding that increasing the frequencies of the detection signal(i.e. a frequency difference between the frequencies of the referenceand responding radiations at the receiver) can be achieved byvarying/increasing either the difference between the optical pathlengths traversed by the reference and inspection radiations untilarriving at the detector (thus varying the time lag between the arrivalsof said radiations at the detector), or by increasing the frequencysweeping rate β of the THz generator. According to the invention,increasing the frequency difference between the reference and inspectionradiation is achieved by providing an optical drive module which isadapted for fast wavelength sweeping of one or both of the DFB lasersfacilitating to achieve higher frequency sweeping rates of the THzgenerator.

As noted above, THz emitters (radiation generators) typically include anoptical drive associated with two or more lasers. THz radiation isgenerated by photomixing of the output beams from the two or more laserssuch that THz radiation has a continuous wave (CW) form with thefrequency equal to the beat frequency (frequency difference) of thelasers' output beams. Typically, at least one of the lasers is a DFBlaser and thus a control over the frequency of the THz radiation, neededfor spectroscopic applications, may be achieved inter alia by utilizingthermo-electric-cooling (TEC) systems for adjusting/controlling thetemperature and thus output wavelength(s) said at least one laser. Inthis manner, the frequency of the output THz radiation can be sweptcontinuously by gradually changing the operating temperature of said atleast one laser (more specifically by changing the temperature of theactive region of the laser diode, e.g. substantially linearly withtime). In many cases, the wavelengths of two DFB lasers of the opticaldrive are swept in opposite directions, e.g. by heating one laser whilecooling the other, thus increasing the rate of sweeping of the THzoutput frequency and the overall frequency sweeping range.

Hence, according to the conventional approach, the frequency sweepingrate β is strongly dependent on the heat pumping rates of the TECsystems and also on the coupling of such TEC systems with the activeregion of the laser diodes. Changing the temperature of a laser is arelatively slow process which rate is limited by the ability of the TECsystems to pump heat from the active region of the laser diode (which issmall relatively to the TEC system). This, in turn, practically limitsthe frequency sweeping rate β up to the order of 1 (THz/Sec) even whengood TEC systems are used.

According to the invention, sweeping of the laser(s)'wavelength/frequency may be performed by utilizing temperaturevariations of the lasers active region as well as by varying/modulatingother operational parameters of the optical drive (or of the lasers) toobtain frequency modulated continuous wave (FMCW) outputsignal/light-beam from the optical drive. The frequency (the baseline)of the signal is swept gradually by the temperature variation while thefrequency modulation can be achieved for example by modulating thecurrent through the laser diode to affect its output wavelength or byutilizing electro-optical in the path of the output beam of the laserfor modulating its wavelength.

Indeed, the common techniques for exercising variation of the outputwavelength of a light source/laser include controlling/adjustment of thetemperature and/or the current of the light source. However, it shouldbe noted that some aspects of the invention, and specifically thoseaspects relating to the utilization of fast frequency sweeping rates forthe purpose of reducing measurement noise or improving the rangeresolution (depth resolution), are not limited to the specific techniqueby which fast frequency sweeping rates are obtained. Accordingly, othertechniques, which are currently known or which will be applicable in thefuture, for varying the output wavelength/frequency of light sourcemight also be used for implementing the technique of the presentinvention and providing high rate frequency sweeping and/or modulatedfrequency sweeping without departing from the scope of the presentinvention.

Thus, according to one broad aspect of the invention, there is provideda method for use in spectroscopic measurements of a sample. the methodcomprising: generating inspecting and reference electro-magneticradiation components of substantially the same frequency contents beingswept according to a predetermined frequency pattern, directing saidinspecting and reference radiation components to a detector along firstand second different paths respectively, the sample being located in thefirst path (allowing interaction of the inspecting radiation componentwith a sample) to thereby induce a frequency difference (e.g. apredetermined frequency difference) between a frequency of theinspecting radiation component and the reference radiation componentinteracting at the detector. A signal resulting from the interactionbetween the inspecting and reference radiation components is thusindicative of one or more properties of the sample at a location wherethe inspecting radiation interacts with the sample.

According to some embodiments of the invention the frequency differencebetween a frequency of the inspecting radiation component and thereference radiation component interacting at the detector, is induced bycontrolling at least one of the predetermined pattern and thepropagation of the inspecting and reference radiation components to thedetector. Also, the predetermined frequency pattern may be selected inorder to provide at least one of the following: (i) the frequencydifference between the reference and inspecting radiation component atthe detector, being highly sensitive to a difference between said firstand second paths thereby increasing spatial resolution of detection of adepth location of the interaction between the inspecting radiationcomponent and the sample; and (ii) the frequency difference between thereference and inspecting radiation component at the detector, beingwithin a certain frequency range thereby increasing signal to noiseratio of detection of said one or more properties of the sample.

Preferably, at least one of the inspecting and reference opticalradiation components is formed by one or more pairs of interacting lightbeams. The frequency of the at least one respective radiation componentis thus a beat frequency of said interaction.

The controlling of the propagation of the inspecting and referenceradiation components to the detector is such as to allow freepropagation of the reference radiation component to the detector (namelypropagation independent of a propagation time of the inspectingradiation to the detector), thereby inducing said predeterminedfrequency difference and enabling to desirably increase said frequencydifference to thereby increasing signal to noise of the measurements.

The controlling of the pattern of the beat frequency sweeping comprisesconcurrently affecting a first, global frequency sweeping rate during acertain time period and a local modulation of the frequency sweepingwith a second higher sweeping rate.

According to another broad aspect of the invention, there is provided amethod for electromagnetic frequency sweeping of output light from alight source comprising one or more laser diodes, the method comprising:gradually changing an operational temperature of an active region of atleast one laser diode thereby causing a substantially monotonic changein the frequency output of the laser diode; and concurrently modulatingan electric current through at least one of the laser diodes therebyinducing additional frequency sweeping pattern in the frequency outputof the laser diode.

Preferably, a first characteristic time scale of said monotonic changein the frequency output is longer than a second characteristic timescale of the frequency modulation. The frequency modulation therebypresents a sequence of local changes in the frequency output during aglobal change corresponding to said monotonic change in the frequencyoutput.

According to another broad aspect of the invention, there is provided amethod for use in frequency modulated continuous wave (FMCW)spectroscopy, the method comprising producing FMCW electromagneticradiation by interacting light beam output from at least two laserdiodes and gradually changing an operational temperature of an activeregion of at least one of said laser diodes thereby causing asubstantially monotonic change in the frequency output of said at leastone laser diode and concurrently modulating an electric current throughat least one of the laser diodes for inducing a frequency modulation inthe frequency output of the laser diode, thereby increasing a span offrequency gradient of said electromagnetic radiation during themeasurements allowing higher signal-to-noise ratio of the measurements.

An operative frequency of the FMCW spectroscopy may be in a THz regime.The interaction of the light beams from said at least two laser diodesgenerates at least one FMCW electromagnetic radiation beam in a near THzfrequency range.

More specifically, in some embodiments of the invention, the methodincludes: (i) irradiating the sample with an incident beam being a firstTHz-range FMCW beam to cause a THz radiation response of the sample;(ii) causing an interaction between the response beam of the sample anda certain reference beam being a second THz-range FMCW beam time shiftedfrom the corresponding first FMCW beam, and (iii) detecting anelectromagnetic signal resulting from said interaction and having afrequency corresponding to the time shift between the first and secondbeams and to said frequency modulation of the laser diode.

Generation of the at least one FMCW electromagnetic radiation beam in aTHz frequency range utilizes generation of said incident and referencebeams, while performing continuous frequency sweeping with certainsweeping rate β. The parameter β is controlled by temperature variationof at least one of the lasers or by current modulation induced in atleast one of the lasers, or preferably by combination of both thetemperature and current variations. Temperature variation is arelatively slow process, while the current modulation, which may beachieved at electronic speeds, is a quicker one. For example, the scalefactor pertaining to temperature controlled frequency variation appliedto a laser with wavelength of about 800 nm is approximately 30GHz/deg.K. Utilizing the electric current modulation, the scale factorrelating frequency variation to laser drive current is approximately 1.6Ghz per mA.

According to the invention, “slow” temperature variation may be used forspectroscopic coverage, while fast current modulation may be usedsimultaneously to on the one hand improve the radial resolution (depthresolution) of the spectroscopic measurements beyond that achievablewith slow frequency sweeping rates (e.g. utilizing frequency sweepingbased temperature control alone), and on the other hand improve thesignal to noise of the measurement due to higher intermediate homodynefrequency. In this case the laser will be driven by modulated currentwaveform (e.g. sinusoidal/saw-tooth/triangular etc'), while thetemperature may be varied simultaneously in a linear fashion.

According to yet further broad aspect of the invention, there isprovided a method for use in spectroscopic measurements of a sample, themethod comprising: generating inspecting and reference radiationcomponents corresponding to respectively first and second pairs of lightbeams of the same beat frequency contents being swept according to apredetermined pattern and directing said inspecting and referenceradiation components to a detector along first and second differentpaths, the sample being located in the first path, said pattern beingselected so as to induce a desired frequency difference between afrequency of the inspecting radiation component and the referenceradiation component interacting at the detector.

The invention also provides a spectroscopic measurement methodcomprising: generating inspecting and reference radiation componentscorresponding to respectively first and second pairs of light beams ofthe same beat frequency contents being swept with a certain sweepingrate, and directing said inspecting radiation component to propagate toa detector along a first path passing through a sample and directing thereference radiation component to the detector along a second path, thefirst and second paths being such that the inspecting and referenceradiation components interacting at the detector correspond to lightbeam pairs generated at different times thereby inducing a desiredfrequency difference between the interacting inspecting and referenceradiation components.

According to yet another aspect of the invention, there is provided asystem for use in spectroscopic measurements of a sample, the systemcomprising: a radiation transmitter unit configured and operable forgenerating inspecting and reference electro-magnetic radiationcomponents (e.g. optical or quasi-optical or THz range radiation) ofsubstantially the same frequency contents, and for sweeping saidfrequency according to a predetermined frequency pattern; and a detectorlocated in a first path of the inspecting radiation components afterpassing through a sample and in a second path of the reference radiationcomponent directly propagating from the transmitter unit to therebyinduce a frequency difference (e.g. being predetermined difference)between a frequency of the inspecting radiation component and thereference radiation component interacting at the detector, a signalresulting of interaction between said inspecting and referencecomponents being indicative of one or more properties of the sample at alocation where said inspecting radiation interacts with the sample.

According to some embodiments of the invention the system is configuredto adjust/tune/control the frequency difference between the componentsof the inspecting and the reference radiation at the detector, bycontrolling at least one of the predetermined frequency pattern and thepropagation of the inspecting and reference radiation components to thedetector. Additionally or alternatively the predetermined frequencypattern may be selected such that the frequency difference, between theinspecting and reference radiation components at the detector, is highlysensitive to a difference between the first and second paths (therebyincreasing spatial resolution of detection of a depth location of thesample portion being inspected) and/or it is within a certain frequencyrange which is selected in order to increase the signal to noise ratioof detection of one or more properties of the sample.

According to yet another broad aspect of the invention there is provideda system for sweeping of the output frequency of a light sourcecomprising one or more laser diodes, the system comprising:

a frequency sweeping module adapted for affecting gradual change of oneor more operational parameters of a light source to thereby causegradual sweeping of the frequency of the light source across a certainfrequency range; and

a frequency modulation module adapted for modulating one or moreoperational parameters of the light source to induce modulation in thefrequency of light source.

Said one or more laser diodes may comprise one or more DFB lasers; andsaid gradual change of said one or more operational parameters maycomprises a gradual change of the operational temperature of an activeregion of at least one DFB laser affecting substantially monotonicsweeping of the frequency of said at least one DFB laser. The frequencysweeping module may comprise at least one temperature control unitconnectable with at least one TEC system thermally coupled with said atleast one DFB laser; said temperature control unit is configured andoperable for controlling the operation said at least one TEC system.

The frequency modulation module may comprising at least one currentcontrol unit connectable to at least one laser diode and configured andoperable for modulating an electric current flowing through said atleast one laser diode to thereby induce modulation in the frequency ofsaid at least one laser diode.

Output radiation from said light source may be obtained by couplinglight beams from said one or more laser diodes. The light source maycomprise two laser diodes and the frequency sweeping module may includeat least one temperature control unit. For example, two temperaturecontrol units may be used and may be associated respectively with twolaser diodes; the frequency sweeping module is adapted in such case tooperate said two temperature control units to change the temperatures ofthe laser diodes in opposite directions.

The frequency modulation module may also include one or more currentcontrol units associated with at least one of the laser diodes. Forexample, two current control units associated respectively with twolaser diodes. The frequency modulation module may be adapted to operatesaid two current control units to modulate the currents through therespective laser diodes in opposite directions.

As indicated above, in some embodiments of the invention, a firstcharacteristic frequency variation rate in the output frequency of thelight source obtained by operating said frequency sweeping module islower than a second characteristic frequency variation rate obtained byoperating said frequency modulation module. The modulation in thefrequency of light source thus presents a sequence of local changes inthe frequency output during a global change corresponding to saidgradual sweeping in the frequency output.

The invention in its yet another aspect provides a high-frequencyspectroscopy system, the system comprising:

a radiation generator for generating an inspecting radiation and areference radiation of the same properties;

a frequency sweeping module associated with said radiation generator forinducing frequency modulation in said inspecting and reference radiationcomponents, said frequency modulation having a global frequency sweepingrate and a local frequency sweeping rate corresponding to desiredfrequency and radial resolution to be obtained in a spectroscopicmeasurement.

Such system comprises or is connectable to a radiation receiver unitconfigured and operable for mixing the reference radiation component anda responding radiation component being a reflection or transmission ofthe inspecting radiation component from or through the sample. Thereceiver unit is configured and operable for determining a frequencydifference between the reference and responding radiation componentsbeing mixed and utilizing said local frequency sweeping rate to identifya in-depth location, at said radial resolution of a sample, associatedwith said received responding signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, embodiments will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 shows an example of an antenna system, for use in spectroscopyaccording to the conventional approach;

FIG. 2 illustrates noise density characteristics of an amplifier whichcorresponds to the flicker noise effect;

FIG. 3 illustrates schematically a THz spectroscopy module according tothe invention.

FIG. 4 is a flow diagram of a method according to the invention forsweeping the output wavelength of a light source such as DFB laser.

FIGS. 5A and 5B illustrate graphically two time profiles of THzfrequency sweeping, where FIG. 5A shows the THz frequency sweepingobtained by changing the temperature of the light source from which theTHz radiation is generated, and FIG. 5B is a time profile of FMCWradiation obtained according to the invention by sweeping the THzfrequency while changing both the temperature and the current of thelight source.

FIG. 6. is an example of a light coupling assembly used in the presentinvention for receiving and mixing light beams from two lasers andproviding two output light beams having substantially similar spectralcontents and energy.

FIGS. 7A to 7D show few examples of possible current modulation schemeswhich can be used in accordance with the present invention forgenerating frequency modulated output laser beams.

FIGS. 8A to 8C describe the delay-induced frequency offset problemresulting from the conventional approaches illustrated in FIGS. 1, 3 and4.

FIG. 9 exemplifies a signal detection process according to someembodiments of the invention for implanting the depth profiling.

FIG. 10 illustrates schematically an arrangement suitable for depthprofiling of the invention.

FIG. 11 shows an example of the invention for achieving global and localfrequency sweeping rates.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is made to FIG. 3 illustrating an example of a THz transceiver100 according to an embodiment of the present invention including a THztransmitter 110 and a detector (receiver) 120. In the present example,the transceiver 100 is configured and operable for carrying out accuratespectroscopic measurements of a sample/object in the THz frequencyregime. The THz transmitter 110 is adapted to generate THz radiationwith high frequency-sweeping rates β, and to transmit at least a part ofthis THz radiation, inspecting radiation IN, towards the object/sample Ounder inspection, and reference radiation RR towards the THz detector.The THz detector 120 is capable of receiving and detectinghigh-frequency modulated signals, being therefore capable of exploitingthe high frequency-sweeping rate β for generating a detection signal HS(e.g. homodyne signal) of relatively high frequencies indicative of theresponse radiation RS emanating from/through the object in response toits irradiation. The relatively high frequencies of the signal HS resultwith high signal to noise ratio and with improved range resolution ofthe spectroscopic measurements.

The THz transmitter 110 includes an optical drive (light source system)OD and a THz emitter EM optically coupled thereto. The THz emitter EM isconfigured and operable for generating THz radiation by mixing inputlight beams, which are light signals emanating from the optical driveOD. The optical drive includes at least two light sources, generallydesignated L₁-L_(n), which may be light emitting elements themselves orlight input ports associated with remotely located light emittingelements (e.g. via optical fibers), and also includes an optical drivecontroller ODC. The optical drive OD is configured and operable togenerate at least two light beams, LB₁ and LB₂ (typically in the IRwavelength range), which are directed onto an active region of the THzemitter EM (which serves as a photomixer), and thus generate an electriccurrent/EM-field in the THz band.

The optical drive controller ODC is configured and operable forcontrolling the operational parameters of at least one light source(e.g. L₁ or L₂) such as to allow high rate wavelength sweeping of the atleast one output light beam (e.g. LB₁ or LB₂). Utilizing the opticaldrive controller ODC, the transmitter 110 is capable of sweeping thefrequency of the transmitted radiation with high frequency-sweeping rateβ and across a desired THz frequency range suitable for spectroscopicmeasurements.

Detector 120 is configured for receiving and detecting (e.g. homodynedetection) of the radiation response RS by utilizing mixing of theradiation response RS with reference radiation RR which is received fromthe transmitter 110 (e.g. directly). As a result of such mixing, anoutput (homodyne/intermediate) signal is generated, by the detector,containing intermediate frequencies of the differences between thefrequencies of the mixed reference radiation RR and radiation responseRS.

As noted above, with the conventional approach for executing frequencysweeping of the laser diode output, i.e. by temperature changes of theactive region of the diode, it is impractical to provide a constant highsweeping rate β of the high-frequency signal across the full frequencyrange of THz spectroscopy. However, performing THz spectroscopy withhigh frequency sweeping rates β would be advantageous in terms ofmeasurement/detection accuracy.

The present invention provides a solution for the above by utilizing afirst, substantially steady/monotonic sweeping of the THz radiation witha frequency sweeping rate β₀ and utilizing a second modulated sweepingwith higher rate β₁. Thus, according to the invention an effectivemodulated frequency sweeping rate β can be obtained for example in theform ofβ=β₀+β₁(t)where t is a time parameter, β₁(t) is the sweeping rate which is anon-linear function of time to generate desired sweep rate alternationduring said monotonic sweeping with the rate β₀.

This can be achieved by configuring the optical drive controller ODCwith the ability to apply a monotonic/constant wavelength sweeping rateto one or more of the light sources L₁-L_(n) and to apply an additional,modulated wavelength sweeping to at least one of the light sources,which may be the same or different from said at least one light source.To this end, the optical drive controller ODC includes a frequencysweeping controller FS configured to induce a first monotonic sweepingof the output wavelength from one or more of the light sources L₁-L_(n)by controlling at least some of their operating parameters and afrequency modulation controller FM affecting a modulation of the outputbeam wavelength of (said) one or more of the light sources L₁-L_(n) bycontrolling the same or different parameters of their operation. Itshould be noted that the operating parameter(s) of the light source tobe controlled may be that of the light emitter itself or of the lightinput port and/or associated light guide (generally light propagationmedia).

As noted above, the transmitter is configured and operable forgenerating reference radiation RR and transmitting it towards thedetector. The reference radiation RR is mixed at the detector with theradiation response RS from the object which results in the detectoroutput signal HS (being the homodyne/intermediate frequency signal).

The reference radiation RR typically includes a portion of the lightbeam(s) emerging from the optical drive OD. Reference and inspectingradiation portions are obtained by splitting the light beams from theoptical drive OD into the reference radiation portion and the inspectingradiation portion and directing the reference portion to the detector120 and the inspecting radiation portion towards the emitter. In thiscase, THz frequency electric field (i.e. reference oscillator) isgenerated at the detector 120 by mixing the light beams of the referenceradiation portion.

Generally the reference radiation RR and the inspecting radiation IN aresourced concurrently from the same origin (e.g. by light beams from theoptical driver OD or THz radiation from the emitter EM). Accordingly, atthe time these radiations are generated/emitted from the transmitter110, they are associated with similar THz content (frequencies/modes).It should be understood that THz content refers to the frequencies/modesand possibly also the respective intensities which are included in thereference RR and in the inspecting IN radiation or which can begenerated therefrom, e.g. by mixing.

However, the portions of the inspecting IN and reference RR radiationwhich arrive concurrently to the detector 120, correspond to light beamsoriginated at different times from the transmitter 120 and are thusassociated with different THz content (i.e. because there may be a timedelay τ between arrivals of concurrently generated beams at thetransmitter due to a difference ΔR in the length of their optical pathto the detector). This different THz content of the inspecting IN andreference RR radiation gives rise (or at least increases) thefrequencies of the output signal HS which is obtained after mixing ofthose radiations at the detector.

The difference in the frequency contents of the reference RR andinspecting IN radiations, and accordingly the frequency of the outputsignal HS, is of the order of the frequency sweeping rate β multipliedby the time delay τ, i.e. ˜β·τ. Frequency sweep due to temperaturevariation is fairly coarse and achieves relatively big frequency changeover the temperature range but fairly slow. Considering frequencysweeping rates β₀ of this order and considering a difference ΔR betweenthe optical paths of the reference RR and inspecting IN radiation ofabout few meters (the inspecting radiation IN propagates 1 m from thetransmitter 110 to the object O and 1 m from the object O to thedetector 120 while the reference radiation RR propagates a negligibledistance), a time delay τ of a few nanoseconds is obtained between thereference RR and inspecting IN radiation and accordingly frequencies ofthe order of tens of KHz are obtained in the output signal.

As noted above, accurate spectroscopic measurement, namely having highSNR in the output signal HS and/or high range resolution of themeasurements, can be obtained when the output (homodyne) signal HS is ofrelatively high output frequencies, e.g. in the range of hundreds of KHzand up to few MHz and above. The optical drive controller ODC of theinvention facilitates high sweeping rates β of the THz radiation fromthe transmitter by controlling/modulating one or more operationalparameters of the light sources L₁-L₂ such as their operating currentand temperatures. As a result, an output signal HS of higherintermediate frequencies can be generated at the detector, and thusaccurate spectroscopic measurements in the THz regime can be obtained.This will be described more specifically further below.

Generally, conventional DFB lasers have frequency coverage of about 1.5THz (the output frequency of the laser can be swept by about 1.5 THz).Accordingly, photomixing the output light beams from two DFB lasersallows generating THz radiation which can be swept to cover a range ofabout 3 THz.

According to the invention, more than two light sources/lasers might beeffectively utilized for providing spectroscopic measurements with broadfrequency coverage in the THz regime. In this case, at least two of themultiple lasers have different frequency output ranges. By photomixingdifferent pairs of lasers (e.g. successively) while sweeping the outputfrequencies of each photomixed pair, different frequency ranges in theTHz regime can be covered thus providing a broader total frequencycoverage.

In the embodiment of the invention illustrated in FIG. 3, an optionalfrequency coverage controller FC is included being configured andoperable for selecting and operating different pairs of the lightsources L1-Ln successively, resulting in different beat frequencies.Those different beat frequencies are then swept by the utilizing atleast one of the frequency sweeping and frequency modulation controllersto cover different THz ranges (optionally complementary ranges) therebyallowing THz spectroscopy within a broad frequency/spectral range.

For example, utilizing three DFB lasers, photomixing of the first andsecond lasers can be used to sweep the beat frequency (i.e. the THzfrequency) within a first THz range which may be about 3 THz wide. Then,the first laser may be photomixed with a third laser, having outputfrequency range different from the second laser, and thus the resultingbeat frequency can be swept within a second THz range different from thefirst THz range (first and second ranges being possibly complementaryranges). As also the second range may have width of up to about 3 THz,total frequency coverage in the THZ regime of about 6 THz can beobtained. Even broader frequency coverage can be obtained for example byutilizing additional lasers (more than three) and by coupling differentpairs of these lasers at each specific time period to allow sweeping ofthe beat frequency within multiple THz ranges.

Reference is made to FIG. 4 exemplifying a flow diagram 200 of a methodaccording to the invention for controlling the operation of one or morelaser diodes, such as DFB lasers, to generate an output laser beam withfast variation of its wavelength. The method can be implemented in anoptical drive OD (i.e. by the optical drive controller ODC) illustratedin FIG. 3 and can be used to facilitate the generation of frequencymodulated continuous wave (FMCW) having high rate of frequencysweeping/variation THz by THz generators/emitters.

Wavelength/frequency sweeping of a laser diode output with high sweepingrates is achieved according to this method by concurrently and/orinterchangeably carrying out the following operations:

In a first operation 210, at least one operational parameter of thelaser diode, such as its operational temperature (e.g. the temperatureof its active region) is controlled (controllably varied) formaintaining continuous sweeping 210 the laser diode wavelength forexample for providing a monotonic/steady wavelength sweeping withrelatively fixed sweeping rate. With respect to the system of FIG. 3,this operation might be performed by the frequency sweeping controllerFS to control the operational temperatures of one or more of the lasersfor example by controlling the operation of thermo-electric cooling(TEC) systems (TEC) coupled therewith.

In a second operation 220, the same or other parameter of the laser'soperation is controlled for modulating the laser's wavelength in time.This can be for example achieved by applying modulation to the currentthrough the laser diode thus affecting a modulation of its output. Withreference to the FIG. 3 this operation might be performed by thefrequency modulation controller FM.

By changing the operational temperature of a DFB laser, its outputfrequency can be changed at a rate of about 1.5 THz/sec. This can beachieved for example by heating/cooling the lasers utilizing a TECsystem with high heat pumping rate (for example the TEC system disclosedin a co-pending U.S. application Ser. No. 61/292,649).

FIG. 5A illustrates graphically the sweeping of THz radiation frequencyas obtained by photomixing of light outputs of two DFB lasers whichoperational temperatures are changed in time in opposite directions.Graph G1 illustrates the evolution of THz frequency as function of timewhile sweeping of the THz radiation frequency across a range of about300 GHz-3.5 THz. The slop of graph G1 designates the sweeping rate βwhich is substantially constant in this case. Frequency sweeping isobtained by applying heating and cooling respectively to the two DFBlasers (their active regions) such that their wavelengths are swept toopposite directions. Since the temperature variation of the lasers is agradual and relatively slow process, relatively low frequency sweepingrate β=˜3 THz/sec is obtained.

Turning back to FIG. 4, in order to increase the frequency sweepingrates and to enable accurate spectroscopy measurements, the secondoperation 220 is carried out for modulating the laser's wavelength intime and thus temporally inducing high frequency sweeping rates of theTHz radiation. For example, in addition to the continuous frequencysweeping carried out in the first operation by changing the DFB laser/stemperature, in the second operation the wavelength of one or both ofthe DFB laser/s is fast modulated by changing the electric current forthe laser/s.

The electric current change of the DFB laser has an immediate affect onthe lasers' output (as opposed to temperature changes which requirestime for cooling/heating the lasers active region) and thus higherfrequency modulation rate can be achieved corresponding to wavelengthvariation rate of up to the order of 100 nm/sec. By exploiting the highwavelength modulation rates in the lasers' output, THz sweeping withfrequency sweeping rates of about β=˜15 GHz/milisec can be obtained.This is about ten times higher that the frequency modulation obtainedsolely by the temperature variation.

However, only a limited variation of about 0.1 nm of the wavelength ofthe DFB laser is obtained by the change of the electric through thelaser, which is insufficient for generating and sweeping across thewhole THz frequency range (zone). Thus according to the invention, thetemperature variation of the laser diode (e.g. first operation 210) canbe used to provide substantially monotonic/constant THz sweeping withtypical rates of e.g. β₀=˜3 THz/sec while current modulation is applied(e.g. second step 220) for providing alternating THz sweeping rates inthe range of β₀=˜+/−30 THz/sec.

It should be understood that applying a fast modulation of the laserwavelength is not limited to tuning/modulation of the electric currentthrough the lasers and it can be performed for example applyingadditional fast and accurate temperature change/modulation, in additionto the sweeping applied by the temperature. Alternatively oradditionally, modulation of the wavelengths of the laser beams can beperformed by affecting the optical path of the laser beams for exampleby utilizing a non-linear optical element along the optical path. Tothis end, the term operational parameters of the light sources/lasersinclude also the optical path/medium which the light beams from thoselight sources traverse. Yet another option is to use a mechanical,optical or any element to frequency-modulate the output beam from theTHz emitter.

Turning now to FIG. 5B, there is shown a graphic illustration G2 of theTHz frequency vs. time as generated utilizing a frequency sweepingtechnique according to an embodiment of the present invention. In thisexample the THz frequency is swept from low frequencies to highfrequency (or vice versa within frequency sweeping range of about 300GHz-3.5 THz) using a gradual temperature change while concurrentlyrelatively fast modulation of electric current through the activeregions of the lasers is applied. Similarly to the graph G1 of FIG. 5Aalso here, the gradual temperature change provides monotonic frequencysweeping across the desired THz frequency range with monotonic sweepingrate β₀ of about ˜3 THz/sec. A fast modulation of the frequency withperiod t_(m) of about 1 msec and with relatively low frequency shiftingamplitude of about 30 GHz is obtained by applying current modulation tothe laser diode(s). This results with relatively high frequency weepingrates β₁ ranging/alternating in between +/−30 THz/sec.

In this example, current modulation if applied to both laser diodes withtime shift of about 0.5 msec (i.e. phase shift of about π) between thecurrent modulations such that when relatively high current is flowingthrough one of the laser diodes, relatively low current flows thoroughthe other. This results with the output wavelengths of the laser diodesswaying in opposite directions thus increasing the resultant frequencyshifting amplitudes. It should be noted however that according to theinvention, each of the electric current modulation and the temperaturevariation can be applied to only one of the laser diodes and notnecessarily to the same one.

A comparison of the THz frequency sweeping (and the rates) illustratedin FIGS. 5A and 5B yields the following results: Without frequencymodulation (e.g. without modulating the current), and considering timedelay τ of about 7 nanoseconds between the reference and inspectingradiations (e.g. corresponding to length difference ΔR of about 2 mbetween the optical paths of the reference and inspecting radiationbetween transmitter to the detector) the frequency of the output(homodyne) signal of the detector (e.g. the frequency difference betweenthe reference inspecting radiation) and is about 20 KHz. With frequencymodulation, e.g. when current modulation is applied, β approaches 30GHz/msec and the frequency of the output signal of the detector reachesto about 200 KHz.

Reference is made to FIG. 6 illustrating a more specific, but notlimiting example of THz spectroscopic system 100A according to theinvention. Similar referenced numbers are used in all the figures todesignate common elements having essentially similar functionality orpurpose.

System 100A includes a THz transmitter (THz radiation generator) 110 anda detector 120. The radiation generator 110 includes a THz emitter ENand an optical drive OD optically coupled together through an opticalcoupling OC for generating THz radiation which can be used to irradiatean inspected object O with inspecting radiation IN. The optical drive ODincludes, in the present example, two light sources L1 and L2 (DFBlasers) associated with respective thermo-electric cooling systems TEC1and TEC2 and an optical drive controller ODC connected to the lightsources and to the thermo-electric cooling systems. The ODC isconfigured and operable for controlling the temperatures of—and theelectric currents through—the light sources L1 and L2 and to therebycontrol and vary the wavelength of the lasers' output beams LB1 and LB2with relatively fast rates. As noted above, this enables sweeping theTHz radiation generated, at the emitter EM by photomixing of those lightbeams, with high frequency sweeping rates β.

To this end, the optical drive controller ODC includes afrequency/wavelength modulation controller unit FM, which in thisexample include one or more electric current controllers CC(s)connectable to one or more light sources L1 and L2 and configured andoperable for modulating the current through the light sources L1 and L2to affect a modulation of their output wavelengths. The optical drivecontroller ODC also includes a frequency sweeping controller FS which,in this case, includes one or more temperature control unit TC(s) thatare configured and operable for controlling respectively the operationof the thereto-electric cooling systems TEC1 and/or TEC2 and to therebyaffect the temperature of lasers L1 and/or L2 and to allow monotonicsweeping of their wavelengths and of the THz radiation obtained by theirmixing.

The output light beams LB1 and LB2 from the lasers L1 and L2 are mixedtogether and optically coupled with at least one THz emitter EM fromwhich the inspecting radiation and possibly also the reference radiationare generated. In many cases, it is preferable that the mixed lightbeams LB1 and LB2 are split (e.g. by optical coupler OC) into twoportions OL1 and OL2, preferably of substantially similar spectralcontent and energy such that one portion is associated with thegeneration of the inspecting THz radiation and the other is associatedwith or is serving as the reference radiation.

As illustrated in the figure, one THz emitter EM may be included in thetransmitter 110 for generating the inspecting radiation from one portionOL1 of the mixed light beams while another portion of the light beamsOL2 serves as the reference radiation and is transmitted/directed to thedetector where it is mixed to generate a reference oscillator.

In general THz emitter EM may include any suitable photomixer which canbe coupled with an appropriate THz antenna for generating, in theantenna, electric currents having frequencies in THz regime (being thebeat frequency of the two lasers). Known in the art THz emitters utilizephotoconductive semiconductors such as GA to generate THz currents orare based on the free-charge-propagation technology (e.g. vacuum basedtechnology) as disclosed for example in WO 2007/132459 assigned to theassignee of the present invention.

Hence the THz generator/transmitter 110 generates and transmitsreference radiation RR towards the detector 120 which may include (or beconstituted by) a portion e.g. OL2 of the light beams. The detector 120includes a receiver mixer RM adapted for mixing a response radiation RSfrom the object O (referred to herein as being a part of the inspectingradiation IN returned from the object to the detector) with a referenceradiation RR that is transmitted directly from the THz transmitter 110.The receiver mixer RM is configured for carrying out homodyne detectionof the response radiation RS and for generating detection signal HS(intermediate frequency signal) including a beat frequency of theresponse RS and reference RR radiations. The current modulation appliedby frequency modulation controller FM to the laser diode increases thefrequency sweeping rate of the transmitter 110 and thus causes thefrequency difference at the detector/receiver to increase (compared tothe case of no current modulation is applied).

Due to the high frequency sweeping rates β provided by the optical driveof the present invention, the detection signal obtained has relativelyhigh intermediate frequencies allowing accurate spectroscopicmeasurements with relatively high signal to noise ratio over fairlybroad spectral range.

Reference is made to FIGS. 7A to 7D illustrating graphically variousforms of fast current modulations that can be applied to one or more ofthe light sources (lasers) of the systems illustrated in FIGS. 3, 6 inorder to modulate their output wavelengths.

FIG. 7A exemplifies a triangle current modulation waveform where thecurrent I is periodically increased above certain baseline value I₀ bycurrent modulation amplitude I_(m) and decreased back towards thebaseline value I₀. In this example and the increase and decrease ratesas well as the current modulation period t_(m) are fixed constants.

Utilizing current modulation, as illustrated in this figure, THzfrequency sweeping with rates upto 30 THz/sec can be obtained. Forexample, current modulation can be used for modulating the frequency ofa THz radiation which baseline frequency (with respect to which thefrequency modulation is applied) is monotonically swept (e.g. utilizingtemperature variation) with rate of about +3 THz/sec. As a result, thefrequency sweeping rate β of the THz radiation acquires periodic valuewhich may alternate between about +30 THz/sec to −24 THz/sec.Accordingly, the alternating positive and negative high frequencysweeping rates are obtained which can be exploited by thereceiver/detector for providing measurements with high signal to noise(e.g. with non-zero intermediate frequencies and therefore with lowflicker noise).

FIGS. 7B and 9C show two examples of saw-tooth current modulationwaveforms suited for use in the present invention. In FIG. 9B a periodicincrease of the current above a baseline value I₀ with certain finiteincrease rate is followed by abrupt/immediate decrease of the currentback to the base line level I₀; and vice-versa in FIG. 9C. Suchsaw-tooth current modulation schemes can be exploited for providingsubstantially constant and high frequency sweeping rate β. The sweepingrate β obtained is this case may be considered a constant value which ismaintained along all the frequency sweeping range except for at“singular” time points (e.g. t_(s)) at which abrupt decrease/increase ofthe current to the laser diode is applied. Considering the durations ofthese “singular” time points as being negligible, they may be ignored inthe detection module, thus allowing a homodyne detection to be performedas if a non-modulated and high (e.g. ˜30 THz/sec) frequency sweepingrate β is provided by the THz transmitter.

FIG. 5D 9D illustrates an example of a sinusoidal current modulation ofa laser diode with baseline I₀ amplitude I_(m) and period t_(m).

It should be understood that in accordance with the present inventionother modulations of wavelengths of light beams from the optical drivecan be applied. For example any other form of current modulation can beused as well as modulating the wavelengths of the optical drive byvarying modulating other of its operational parameters such as thetemperature of the lasers or operational parameters of otheroptical/electro-optical means in the path of the laser's beam.

It should be also understood that the disclosed method and systems ofthe present invention is not limited for THz spectroscopy. The frequencymodulated continuous wave FMCW sweeping technique of the invention canbe implemented for high frequency sweeping of electromagnetic radiationin various frequency bands including inter-alia UV, visible, IR andmicrowave. The radiation swept by the FMCW technique of the inventionmay be that emanating from one light source/port or a radiation that isgenerated via photomixing of light beams from two or more light sources.

As noted above, according to some embodiments of the present invention,continuous frequency sweeping with high frequency sweeping rates can beeffectively used for depth profiling (3D imaging) of a sample. In thisconnection, the present invention takes advantage of the frequencyoffset that accrues when a linear frequency scan is used. As indicatedabove, such frequency offset resulting from a delay in the time ofarrival of the responding and reference radiation components to thereceiver antenna, typically leads to the flicker noise. However, theinvention utilizes this effect, rather than trying to reduce it, basedon the understanding of the following.

Referring to FIG. 8A, there is shown an effect of frequency scanningonto a frequency off-set. The latter is proportional to a time delaybetween the reference optical signal in the receiver and the receivedwave. At a scale factor of 1 kHz per nsec per THz/sec, the frequencyoffset makes conventional lock-in detection with a band-pass filterineffective, because the frequency offset means the signal averages tozero, or it can even fall outside the band-pass of the averaging filter.It is not practical to maintain τ=0, if delay is needed for adjustingphase or for delay profiling, so an alternative step-scan method is usedas shown in FIG. 8B. This solves the problem, but at the expense of losttime as each step in the frequency profile has to be allowed time tosettle. Minimising scanning time is highly advantageous in practicalapplications. The step-scan approach wastes valuable time because thetemperature control loop which is used for frequency tuning of thelaser(s) requires appreciable time to settle after a step. A typicalpractical step response is shown in FIG. 8C which illustrates the wastedtime while temperature settles. The time wasted in settling may approachthe time available for measurement.

The present invention takes advantage of the frequency offset thataccrues when a linear frequency scan is used. In order to obtainpractical levels of frequency sweep speed and delay, the frequencyoffset will naturally lie above the flicker noise of the receiveramplifier. This allows, the response at the natural offset frequency tobe measured using Fast Fourier Transform processing. The Fast FourierTransform implements a contiguous bank of band-pass filters. Dependingon sweep rate and delay, the offset frequency will lie within one or asmall number of filters (bandwidths). A conventional interpolationalgorithm, well known in signal processing, may be used to estimate thesignal amplitude over a time interval corresponding to the datacollection time of the Fast Fourier Transform. This time intervaldefines the frequency resolution of the THz measurement in the same wayas the frequency jumps in step-scan (as described above with regard toone of the known techniques), but without the settling-time loss.

It is fundamental that the frequency sweep-rate be chosen to becompatible with the desired integration time and the span of thefrequency space swept in the process of the spectroscopic measurement.

For example, if 1 THz span is swept in 1 second and the requiredintegration time is 1 msec, the FFT collection time for each resolvablemeasurement will be 1 msec and 1000 FFTs will be required to cover the 1THz span at a resolution of 1 GHz. In this example, if the time delay isdesigned to be 10 nsec, the frequency at which data is found in the FFTis 10 kHz. The FFT sampling frequency may be chosen to support theexpected frequency of the data and the FFT size adjusted accordingly.The interpolation algorithm takes care of the fact that there will notbe a harmonic relationship between the FFT sampling frequency and thefrequency at which the measured data appears, so the data will splitover a few FFT bins. This process is illustrated in FIG. 9.

It is recognized in this invention that the process maps delay betweenreference and received signals into a frequency location in the FFT.This means that delay can be measured by observing frequency location.The defining relationship is that τ=f/β, where f is the frequencyobserved in the FFT (i.e. beat frequency at the receiver). Theresolution interval associated with this delay measurement is c/β·T),where c is the speed of light and T is the duration of the coherentprocessing dwell. This recognition is the key to 3D imaging or depthprofiling of the sample, where the two spatial dimensions are obtainedby positioning the transmitter and receiver transducers relative to thetarget object and the third dimension (radial distance) is obtained fromthe position of the signal response in the FFT filter-bank. The delaymeasured is a round trip delay which converts to a radial rangeaccording to the following: R=c·τ/2. It should be noted that preferably,in order to get a phase reference (position in space of a sample)calibration of the free space path prior to the measurements on thesample might be needed.

An arrangement suitable for 3D imaging is illustrated in FIG. 10.According to the invention, parameter, β, which is the linear sweeprate, may be exaggerated, or the processing dwell, T, is chosen toachieve the desired resolution parameter in the radial dimension. Forexample, if (β·T)=30 GHz, the radial resolution (spatial resolution)will be 1 cm and the spectroscopic resolution is 30 GHz. The property ofradial resolution is advantageous in spectroscopic measurement for thepurpose of eliminating multi-path reflections. Such reflections exhibitdelays different from the delay of the wanted target object and henceare gated in the FFT into filters (bandwidths) that are separated fromthe filter containing the desired target response.

The parameter β is controlled by temperature variation of the laser(s)or by current modulation of the laser(s). Temperature variation isrelatively low speed process, while current modulation may be achievedat electronic speeds. The scale factor pertaining to temperature controlon a single laser is approximately 30 GHz/deg. K for lasers near 800 nmwavelength. The scale factor relating frequency variation to laser drivecurrent is approximately 1.6 Ghz per mA.

According to the invention, “slow” temperature variation (gradualtemperature change) may be used for spectroscopic coverage while fastcurrent modulation may be used simultaneously to achieve radialresolution. In this case, the laser(s) will be driven by a saw-tooth ortriangular current waveform while the temperature may be variedsimultaneously in a linear fashion. This is illustrated in FIG. 11.

Thus, the present invention provides a simple and effective solution forhigh-quality spectroscopy in high-frequency applications (e.g. THzapplications). The invention provides for high signal-to-noisespectroscopic measurements and also enables depth profiling or 3Dimaging of the sample under inspection with high resolution in bothspatial and frequency domains.

The invention claimed is:
 1. A method for use in spectroscopicmeasurements of a sample, the method comprising: generating inspectingand reference electro-magnetic radiation components of substantially thesame frequency contents, said inspecting and reference electro-magneticradiation components being swept according to a predetermined frequencypattern, directing said inspecting and reference radiation components toa detector along first and second different paths, respectively,allowing interaction of the inspecting radiation component with a samplelocated in the first path, thereby inducing a frequency differencebetween a frequency content of the inspecting radiation component andthe reference radiation component interacting at the detector a signalresulting from said interaction between said inspecting and referenceradiation components being indicative of one or more properties of thesample at a location where said inspecting radiation interacts with thesample; and controlling at least one of said predetermined pattern andthe propagation of the inspecting and reference radiation components tothe detector; wherein said controlling of said predetermined pattern ofthe frequency sweeping comprises concurrently affecting a firs, globalfrequency sweeping rate during a certain time period and a localmodulation of the frequency sweeping with a second higher sweeping rate.2. The method of claim 1, wherein said predetermined pattern is selectedin order to provide at least one of the following: (i) said frequencydifference being highly sensitive to a difference between said first andsecond paths thereby increasing spatial resolution of detection of adepth location of the interaction between the inspecting radiationcomponent and the sample; and (ii) said frequency difference beingwithin a certain frequency range thereby increasing signal to noiseratio of detection of said one or more properties of the sample.
 3. Themethod of claim 1, wherein at least one of the inspecting and referenceradiation components is formed by at least one pair of interacting lightbeams, said frequency of said at least one of the inspecting andreference radiation components being a beat frequency of saidinteraction.
 4. The method of claim 1, wherein said controlling of thepropagation of the inspecting and reference radiation components to thedetector comprises allowing free propagation of the reference radiationcomponent to the detector independently of a propagation time of theinspecting radiation to the detector, thereby inducing saidpredetermined frequency difference and enabling to increase saidfrequency difference thereby increasing signal to noise of themeasurements.
 5. The method of claim 1, wherein the inspecting andreference radiation components are formed by first and second pairs ofthe light beams of the same beat frequency contents which are producedby splitting output light from a light source comprising at least twolasers.
 6. The method of claim 1, wherein said controlling of thepredetermined pattern of the frequency sweeping comprises graduallychanging an operational temperature of an active region of at least onelaser generating at least one of the inspecting and reference radiationcomponents thereby causing a substantially monotonic change in thefrequency output thereof corresponding to said global frequency sweepingrate, and concurrently modulating an electric current through at leastone laser thereby inducing said local frequency modulation in thefrequency output of the laser diode corresponding to said highersweeping rate.
 7. The method of claim 1, wherein a first characteristictime scale of said monotonic change in the frequency output is longerthan a second characteristic time scale of the frequency modulation,said frequency modulation thereby presenting a sequence of local changesin the frequency output during a global change corresponding to saidmonotonic change in the frequency output.
 8. The method of claim 1,wherein the frequency of the inspecting and reference radiationcomponents being an operative frequency of the inspecting radiationcomponent is in a THz regime.
 9. A system for use in spectroscopicmeasurements of a sample, the system comprising: a radiation transmitterunit configured and operable for generating inspecting and referenceelectro-magnetic radiation components of substantially the samefrequency contents, and for sweeping said frequency according to apredetermined pattern; and a detector located in a first path of theinspecting radiation components after passing through a sample and in asecond path of the reference radiation component directly propagatingfrom the transmitter unit, said system being configured to induce apredetermined frequency difference between a frequency of the inspectingradiation component and the reference radiation component interacting atthe detector such that a signal resulting from said interaction betweenthe inspecting and reference components being indicative of one or moreproperties of the sample at a location where said inspecting radiationinteracts with the sample; wherein said system is further configured andoperable for controlling at least one of said predetermined pattern andthe propagation of the inspecting and reference radiation components tothe detector, and wherein said controlling of said predetermined patternof the frequency sweeping comprises concurrently affecting a first,global frequency sweeping rate during a certain time period and a localmodulation of the frequency sweeping with a second higher sweeping rate.10. The system of claim 9, wherein said predetermined pattern isselected in order to provide at least one of the following: (i) saidfrequency difference being highly sensitive to a difference between saidfirst and second paths thereby increasing spatial resolution ofdetection of a depth location of the interaction between the inspectingradiation component and the sample; and (ii) said frequency differencebeing within a certain frequency range thereby increasing signal tonoise ratio of detection of said one or more properties of the sample.11. The system of claim 9, wherein at least one of the inspecting andreference radiation components is formed by at least one pair ofinteracting light beams; said frequency of said at least one of theinspecting and reference radiation components being a beat frequency ofthe light beams interaction.
 12. The system of claim 9, wherein saidreference radiation component propagate to the detector independently ofa propagation time of the inspecting radiation component to thedetector, thereby inducing said predetermined frequency difference andenabling to increase said frequency difference thereby increasing signalto noise of the measurements.
 13. The system of claim 9 comprising: alight source comprising at least two lasers generating said first andsecond pairs of the light beams of the same beat frequency contents; afrequency sweeping module adapted for affecting gradual change of one ormore operational parameters of a light source to thereby cause gradualsweeping of the frequency of the light source corresponding to saidglobal frequency sweeping rate across a certain frequency range; and afrequency modulation module adapted for modulating one or moreoperational parameters of the light source to induce modulation in thefrequency of light source with said second higher sweeping rate.
 14. Thesystem of claim 13, wherein said at least two lasers comprise DFB lasersand said gradual change of said one or more operational parameterscomprises a gradual change of the operational temperature of an activeregion of at least one DFB laser affecting substantially monotonicsweeping of the frequency of said at least one DFB laser.
 15. The systemof claim 14, wherein said frequency sweeping module comprises at leastone temperature control unit connectable with at least one TEC systemthermally coupled with at least one of the DFB laser; said temperaturecontrol unit is configured and operable for controlling the operation ofsaid at least one TEC system.
 16. The system of claim 13, wherein saidfrequency modulation module comprises at least one current control unitconnectable to at least one laser diode and configured and operable formodulating an electric current flowing through said at least one laserdiode to thereby induce modulation in the frequency of said at least onelaser diode.
 17. The system of claim 13, adapted for operating saidfrequency sweeping module and said frequency modulation moduleconcurrently.
 18. The system of claim 13, wherein the output from saidlight source is obtained by coupling light beams from said at least twolasers.
 19. The system of claim 13, wherein a first characteristicfrequency variation rate in the output frequency of the light sourceobtained by operating said frequency sweeping module is lower than asecond characteristic frequency variation rate obtained by operatingsaid frequency modulation module, said modulation in the frequency oflight source thereby presenting a sequence of local changes in thefrequency output during a global change corresponding to said gradualsweeping in the frequency output.